Introduce run_on_main_thread

This introduces a way for a callback to be run on the main thread.

gdb/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* run-on-main-thread.c: New file.
	* run-on-main-thread.h: New file.
	* unittests/main-thread-selftests.c: New file.
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	main-thread-selftests.c.
	(HFILES_NO_SRCDIR): Add run-on-main-thread.h.
	(COMMON_SFILES): Add run-on-main-thread.c.

Change-Id: I16ef82f0564e9f8a524bdc64cb31df79a988ad9f

gdb/run-on-main-thread.c

@@ -0,0 +1,97 @@
+/* Run a function on the main thread
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "run-on-main-thread.h"
+#include "ser-event.h"
+#if CXX_STD_THREAD
+#include <mutex>
+#endif
+#include "event-loop.h"
+
+/* The serial event used when posting runnables.  */
+
+static struct serial_event *runnable_event;
+
+/* Runnables that have been posted.  */
+
+static std::vector<std::function<void ()>> runnables;
+
+#if CXX_STD_THREAD
+
+/* Mutex to hold when handling RUNNABLE_EVENT or RUNNABLES.  */
+
+static std::mutex runnable_mutex;
+
+#endif
+
+/* Run all the queued runnables.  */
+
+static void
+run_events (int error, gdb_client_data client_data)
+{
+  std::vector<std::function<void ()>> local;
+
+  /* Hold the lock while changing the globals, but not while running
+     the runnables.  */
+  {
+#if CXX_STD_THREAD
+    std::lock_guard<std::mutex> lock (runnable_mutex);
+#endif
+
+    /* Clear the event fd.  Do this before flushing the events list,
+       so that any new event post afterwards is sure to re-awaken the
+       event loop.  */
+    serial_event_clear (runnable_event);
+
+    /* Move the vector in case running a runnable pushes a new
+       runnable.  */
+    local = std::move (runnables);
+  }
+
+  for (auto &item : local)
+    {
+      try
+	{
+	  item ();
+	}
+      catch (...)
+	{
+	  /* Ignore exceptions in the callback.  */
+	}
+    }
+}
+
+/* See run-on-main-thread.h.  */
+
+void
+run_on_main_thread (std::function<void ()> &&func)
+{
+#if CXX_STD_THREAD
+  std::lock_guard<std::mutex> lock (runnable_mutex);
+#endif
+  runnables.emplace_back (std::move (func));
+  serial_event_set (runnable_event);
+}
+
+void
+_initialize_run_on_main_thread ()
+{
+  runnable_event = make_serial_event ();
+  add_file_handler (serial_event_fd (runnable_event), run_events, nullptr);
+}